Our objective is to treat males who have congenital adrenal hyperplasia (CAH) and decreased spermatogenesis with an aromatase inhibitor in order to investigate its effects on hormonal parameters and spermatogenesis. CAH is a family of inherited disorders caused by reduced activity of the enzyme required for cortisol synthesis. Decreased cortisol production increases the secretion of ACTH from the pituitary and increases the production of adrenal androgens through negative feedback. In turn, the increased levels of adrenal androgens are aromatized/converted in glandular (i.e., testes) and extraglandular tissues by the aromatase enzyme and result in elevated estrogen levels. Ideally, the production of adrenal androgens is normalized in CAH patients by glucocorticoid replacement therapy. However, even well controlled CAH patients still manifest the adverse effects (compromised final height, polycystic ovarian disease, male infertility, etc.) of elevated androgens/estrogens. Glucocorticoid therapy does not continually normalize ACTH levels because it lacks the close temporal relationship to ACTH pulses and any adrenal activity will result in greater than normal androgen (and thus estrogen) production. We hypothesize that these elevated estrogen levels affect spermatogenesis in males with CAH through the following mechanisms: (1) Elevated estrogens suppress the hypothalamic-pituitary-gonadal axis through negative feedback. Normal LH/FSH gonadotropin secretion is essential for the initiation and maintenance of testicular function and normal spermatogenesis. Chronically elevated estrogen levels (estradiol) affect testicular morphology and testicular steroidogenesis (a) by suppressing pituitary-gonadal secretion, and (b) by a direct toxic effect of estradiol on testicular tissue resulting in a decrease in testicular testosterone production, decrease number of androgen receptors, and create a further negative imbalance in the testosterone-to-estradiol ratio at the gonadal level; (2) Elevated estrogens adversely affect testicular function including Leydig cell, Sertoli cell and germ cell development as shown in experiments with rodents that have been exposed to excess estrogens; (3) Elevated estrogens cause dysfunction of the efferent ductules and epididymis. Therefore, the overarching question of our study is the following: What degree of positive effect will controlling the conversion/aromatization of elevated adrenal androgens into estrogens by gonadal tissue have on spermatogenesis in CAH males? We propose that inhibiting aromatization of androgens to estrogens with an aromatase inhibitor, will improve testicular function and spermatogenesis by normalizing the estradiol to testosterone ratio at the gonadal level and reversing the negative effects of elevated estrogen on androgen receptors, testicular steroidogenesis and pituitary gonadotropins. Aromatase inhibitors have selective action, are well tolerated by patients, and do not interfere with the production of steroid hormones by other related cytochrome P450-dependent enzymes making it ideal for use in CAH patients. [unreadable] [unreadable]